Rust File examples don't compile [duplicate] - rust

Question

This question already has answers here:
Why do try!() and ? not compile when used in a function that doesn't return Option or Result?
(4 answers)
Closed 5 years ago.
The Rust file examples don't appear compile with Rust 1.18.0.
For example:
use std::fs::File;
use std::io::prelude::*;
fn main() {
let mut file = File::open("foo.txt")?;
let mut contents = String::new();
file.read_to_string(&mut contents)?;
assert_eq!(contents, "Hello, world!");
}
Error log:
rustc 1.18.0 (03fc9d622 2017-06-06)
error[E0277]: the trait bound `(): std::ops::Carrier` is not satisfied
--> <anon>:4:20
|
4 | let mut file = File::open("foo.txt")?;
| ----------------------
| |
| the trait `std::ops::Carrier` is not implemented for `()`
| in this macro invocation
|
= note: required by `std::ops::Carrier::from_error`
error[E0277]: the trait bound `(): std::ops::Carrier` is not satisfied
--> <anon>:6:5
|
6 | file.read_to_string(&mut contents)?;
| -----------------------------------
| |
| the trait `std::ops::Carrier` is not implemented for `()`
| in this macro invocation
|
= note: required by `std::ops::Carrier::from_error`
error: aborting due to 2 previous errors

Answer 1

? is a syntactic sugar that checks a Result: if the result is Err, it is returned as if. If there is no error (aka Ok), the function continue. When you type this:
fn main() {
use std::fs::File;
let _ = File::open("foo.txt")?;
}
that means:
fn main() {
use std::fs::File;
let _ = match File::open("foo.txt") {
Err(e) => return Err(e),
Ok(val) => val,
};
}
Then you understand that for now, you cannot use ? in the main, because main returns unit () and not Result. If you want this stuff to work, you can put it in a function that returns a Result and check it from main:
fn my_stuff() -> std::io::Result<()> {
use std::fs::File;
use std::io::prelude::*;
let mut file = File::open("foo.txt")?;
let mut contents = String::new();
file.read_to_string(&mut contents)?;
// do whatever you want with `contents`
Ok(())
}
fn main() {
if let Err(_) = my_stuff() {
// manage your error
}
}
PS: There is a proposition to make work ? in the main.

Answer 2

They do compile. They just don't compile in a main function like that. If you look at the examples, they all have a big "Run" button on them. Click that and it opens the full, unabridged example on the playpen.
The one you've used above expands to this code:
fn main() {
use std::fs::File;
use std::io::prelude::*;
fn foo() -> std::io::Result<()> {
let mut file = File::open("foo.txt")?;
let mut contents = String::new();
file.read_to_string(&mut contents)?;
assert_eq!(contents, "Hello, world!");
Ok(())
}
}
That code doesn't compile because you've put code that propagates a Result into a function (main in this case) that doesn't return a Result.